The invention concerns an HF-surgery device for cutting and/or coagulating biological tissue, which in operation monitors the patient skin contact of a connected neutral electrode having at least two separate contact surfaces. The HF-surgery device includes at least one parallel resonant circuit arranged in a patient circuit, at least one measurement energy source arranged in a measurement circuit galvanically separated from the patient circuit for producing a measurement voltage/current acting between the contact surfaces of the neutral electrode, and at least one measuring and computing unit which in operation determines by means of the measurement voltage/current a tissue impedance which is representative of the patient skin contact and which acts between the contact surfaces. An HF-current flows in the patient circuit which is closed in the cutting or coagulation operation and the measurement voltage which is independent of the HF-current can be produced in the measurement circuit.
The invention further concerns a method for an HF-surgery device for monitoring the skin contact of a neutral electrode which is electrically connected to the HF-surgery device and which has at least two separate contact surfaces. In the method a measurement voltage/current is produced in a measurement circuit, the measurement voltage/current is transferred from the measurement circuit into a galvanically separated patient circuit in which an HF-current flows in operation in the closed condition, the measurement voltage/current in the patient circuit is passed by way of a parallel resonant circuit and the tissue between the contact surfaces and the tissue impedance representative of the patient skin contact is determined by means of the measurement voltage/current.
HF-surgery devices and methods of the above-indicated kind are known from the state of the art. The high frequency (HF)-surgery devices are used for cutting and/or coagulating biological tissue. An active electrode is usually connected to the HF-surgery device. An active electrode has a small current transmission surface in order to achieve a high level of current density. The active electrode is guided by the physician in the treatment and passes the HF-current into the body of the patient. In addition, one or more neutral electrodes connected to the patient are connected to the HF-surgery device for the return flow of the current.
Neutral electrodes are electrodes of relatively large surface area for application to the body of a patient. They serve to form a return flow path for the high frequency current at such a low current density in the body tissue that unwanted physical effects such as burns are avoided. The neutral electrode was intended to be reliably applied with its entire surface area against the body of the patient. In addition care is to be taken to ensure that secure contact of the neutral electrode is ensured for the entire duration of the high frequency use.
Divided neutral electrodes have at least two contact surfaces which are electrically insulated from each other and which are applied to the skin of the patient in mutually juxtaposed relationship and make electrical contact with the skin. Fitted to the patient, the tissue between the contact surfaces represents an electrical connection whose resistance changes with the size of the connecting area. Instead of a divided neutral electrode with separate contact surfaces it will be appreciated that it is also possible to use two or more neutral electrodes.
Various devices are known which are intended to automatically monitor the skin contact of the neutral electrode and, in the event of inadequate contacting with the body of the patient, generate warning signals and/or switch off the high frequency application.
It is known for an auxiliary or measurement current to be passed by way of the contact surfaces of the divided neutral electrode and monitored. The measurement circuit into which the measurement current is fed is galvanically separated from the patient circuit in which the HF-current flows by way of the active and neutral electrodes during use. The skin contact of the neutral electrode can be monitored by the tissue impedance between the two electrode portions being ascertained and observed. A measuring and computing unit of the HF-surgery device can implement continuous checking of the neutral electrode and deliver suitable warning or switch-off signals.
Methods and apparatuses for contact monitoring of neutral electrodes are known for example from DE 32 49 766, DE 35 44 443 or EP 0 390 937.
DE 197 14 972 discloses an apparatus for contact monitoring of a neutral electrode comprising a resonant circuit that is activated by alternating voltage of varying frequencies and arrange of a resonant frequency and that provides a peak reading detector, which records a peak reading of the alternating voltage occurring in the resonance frequency.
U.S. Pat. No. 7,160,293 discloses a multiple radio-frequency return pad contact detection system that eliminates or minimizes interference or measurement interaction between a plurality of pad pairs by providing a different signal source frequency for each pad contact pair, but a frequency which matches an associated series resonant network frequency.
DE 10 2004 025 613 discloses an apparatus that determines a transient impedance between two parts of a two part neutral electrode.
The known methods and circuit arrangements suffer from the disadvantage that the quality of skin contact monitoring can fluctuate. In particular it has been found that, from impedances of higher than about 130 ohms, which can be caused for example by an increased proportion of fat in the tissue, detectability of an inadequately arranged neutral electrode deteriorates.